The present invention relates to a gas purification method, in particular to a gas purification method capable of purifying feed air by removing water vapor and carbon dioxide contained in the feed air introduced into cryogenic air separation plants.
In separating O2, N2 or the like by means of a cryogenic air separation plant the water vapor and the carbon dioxide contained in the feed air are removed before being solidified in the cryogenic section, which is referred to as xe2x80x9cpre-purificationxe2x80x9d. Recently, since such an apparatus that can gather N2 as a product with a high ratio and remove carbon dioxide up to the level of 0.1 or less vol. ppm and further excels in operation performance is being demanded, methods for adsorbing and removing water vapor and carbon dioxide by using adsorption columns are widely carried out.
As a pre-purification by an adsorption method, a method of the temperature swing adsorption type(TSA method) in which the adsorption/desorption of the object, component are carried out by temperature difference between adsorption and desorption processes, and a method of the pressure swing adsorption type(PSA method) in which the adsorption/desorption of the object component are carried out by pressure difference between adsorption and desorption processes, are known Compared with PSA method, TSA method has an advantage in that since a smaller amount of regeneration gas may be required, product yield is much higher, although it has a disadvantage that a large amount of heat for heating the regeneration gas is consumed. On the other hand, PSA method, in spite of such a disadvantage that the purge ratio(the ratio of the amount of the regeneration gas to the amount of the purified gas) is relatively high, has merit in that heat for heating is not consumed Therefore, the two respective methods have been used separately according to the operation conditions (primarily, pressure of the feed air) of the cryogenic air separation plants.
In order to carry out the gas purification effectively by such adsorption methods, Japanese patent publication No Sho 55-19656, for example, discloses a method in which heating, together with depressurizing, is carried out to give the amount of heat up to the degree sufficient to complement the heat of desorption during the regeneration step. The method, in which the PSA method is added to the TSA method, could be referred to as PTSA method(Pressure Temperature Swing Adsorption method).
In the above publication, where the example mentions a drying of gas, there is no description about the case where water vapor and carbon dioxide are removed simultaneously and there is no quantitative description for the heating amount.
In addition, the Japanese patent laid-open publication No. Sho 55-97221 discloses the case where the temperature of the supplied purge gas in a regeneration step is set to be at least 10xc2x0 C. or higher than that of the adsorption step in the first half part and is set to be equal to or lower than that of the adsorption step in the latter half part. An embodiment of the publication discloses that when the operation was carried out under the conditions that the feed air was supplied at 8 atm/10xc2x0 C. and the purge gas was supplied at 40xc2x0 C. in temperature elevating condition and at 13xc2x0 C. in non-temperature elevating condition, the amount of water vapor was 1.5 ppm and the amount of carbon dioxide was 2.5 ppm in the purified air.
In addition, the Japanese patent laid-open publication No. Sho 55-27034 discloses a method using 3 units of adsorption columns, in which one of the adsorption columns is heated and regenerated while the other two units of adsorption columns are used by switching to PSA method, and then successively switched by the above 2 units of the adsorption columns one after another. That is, in order to make the period of time necessary for the heating and regeneration of the adsorption column conform to the switching cycle of the PSA operation, one of the three columns carries out regeneration, with TSA method process while the other two columns are used by switching to PSA method, so that the column returns to the switching cycle of the PSA method at the point of time that the regeneration process with TSA method ends.
The above publications disclose only the concept of the PSA method combined with heating, without description for the degree of heating, its effect, the relations between the adsorbent used and the degree of heating, and the important elements for the embodiment, and thus further study is required to be carried out.
On the other hand, when water vapor and carbon dioxide in feed air are adsorbed and removed in cryogenic air separation plants, the feed air compressed by an air compressor is cooled up to the normal temperature by after-cooler, then further cooled up to 0-10xc2x0 C. by means of refrigerator using freon or the like and introduced into an absorption column.
With respect to a cooling method of feed air in cryogenic air separation plants, a method of directly cooling the feed air by freon refrigerators is employed in a relatively small apparatus and a method of cooling the feed air by a circulating water circulating a water spray cooling column is employed in a relatively large apparatus. However, even in the case of cooling the feed air by the circulating water, freon refrigerators are frequently used to cool the circulating water.
In recent years, however, due to the tendency of not using freon refrigerators from the view of environmental protection, it is required that the feed air is introduced into an adsorption column without cooling by freon refrigerators. However, without using freon refrigerators, the temperature of the feed air to be supplied into the pre-purification apparatus(adsorption column) becomes the temperature of feed air coming out of an after-cooler(the water temperature plus 5-10xc2x0 C.), which would be above 40xc2x0 C. in summer, resulting in that the amount of saturated water vapor contained in the feed air increases in a large amount. For example, the amount of saturated water vapor at 620 KPa, which is a general feed air purification condition, is about 6 times in the case of 40xc2x0 C. for the air temperature, in contrast to the case of 10xc2x0 C.
Therefore, when water vapor is adsorbed and removed by a water vapor adsorbent such as an activated alumina, the air temperature which passed through the activated alumina would be around 60xc2x0 C. due to the adsorption heat generated by the adsorption. For that reason, the amount of carbon dioxide adsorbed by a carbon dioxide adsorbent such as zeolite would decrease up to about ⅓ compared to the case that the adsorption temperature is 10xc2x0 C. because of the temperature dependency of the amount of adsorption(the reduction of the amount of adsorption in high temperature). Therefore, in order to adsorb and remove the water vapor/carbon dioxide under such a condition, 6 times the amount of activated alumina and 3 times the amount of zeolite would be needed, which, to put it conservatively, results in extreme lowering of economic efficiency.
As a method where water vapor and carbon dioxide are adsorbed and removed adequately with a mere small amount of adsorbent, the time duration of the adsorption process can be shortened, which decreases substantially the amount of water vapor and carbon dioxide which can be introduced into the adsorption column. However, in such a case, if the adsorption/regeneration are performed by TSA method, the time periods allowed for heating to perform the regeneration of the adsorbent or cooling after the heating and regeneration should then be necessarily shortened. Therefore, the regeneration or the cooling of the adsorbent would become insufficient and thus the adsorption and removal of water vapor and carbon dioxide cannot be carried out in many cases. In other words, by merely making the cycle of TSA method short, objective of removing water vapor and carbon dioxide by adsorption method without using freon refrigerators cannot be obtained.
It is accordingly an object of this invention to provide a gas purification method for adsorbing and removing impurities by using PSA method in combination with heating, whereby the step switching time and the relations between the temperature and the amount of the feed gas to be purified and the purge gas used for heating and regeneration could be adequate, so that water vapor and carbon dioxide could be removed in an economical and effective way.
To achieve the above objects, the present invention provides a gas purification method for obtaining a purified gas by passing feed gas containing water vapor and carbon dioxide as impurities through a plurality of adsorption columns which are packed with an adsorbent adsorbing the impurities and can be used switchably and thus removing the impurities, the method comprising the steps of;
(a) an adsorption step of adsorbing and removing water vapor first and then the carbon dioxide by introducing the compressed feed gas into the adsorption column;
(b) a depressurization step of lowering the internal pressure of the adsorption column down to atmospheric pressure after the adsorption step;
(c) a heating step of heating and regenerating the adsorbent by introducing a purge gas not containing the impurities into the adsorption column after the depressurization step; and
(d) a repressurization step of elevating pressure by introducing a gas not containing the impurities into the adsorption column after the heating step;
wherein the respective periods of time of the respective step (a), (b+c) and (d) are set to be within the range of 10 to 40 min.
According to another aspect of the present invention, the temperature of the feed gas is within the range of 30-45xc2x0 C. in the adsorption step, the amount of the purge gas in the heating step is within the range of 0.2-0.5 relative to the amount of the feed gas in the adsorption step, and the amount of heat supplied to the adsorption column by the purge gas is 0.5 or more relative to the amount of heat supplied by the feed gas to the adsorption column in the adsorption step.
According to another aspect of the present invention, the compressed feed gas is cooled by heat-exchanging with air or coolant water or the compressed feed gas is cooled by being subjected to countercurrent gas-liquid contact with circulating coolant water, prior to being introduced into the absorption column.
According to another aspect of the present invention, the purified gas purified in the adsorption column is cooled to the temperature of 60xc2x0 C. or lower by heat-exchanging with air or coolant water, or the compressed feed gas and the purge gas are heat-exchanged so that the compressed feed gas is cooled and the purge gas is heated simultaneously.
According to another aspect of the present invention, the feed gas is the feed air of the cryogenic air separation plants and the purge gas is the exhaust gas discharged from the cryogenic air separation plants.
According to another aspect of the present invention, the adsorption column is packed with the water vapor adsorbent at an inlet side of the feed gas thereof and the carbon dioxide adsorbent at an outlet side of the purified gas thereof, and the weight ratio of the water vapor adsorbent to the entire adsorbent is 0.7 or less.
According to another aspect of the present invention, the water vapor adsorbent is one or a combination of two or more types of the adsorbents selected from the group consisting of activated alumina, silica gel, and A type zeolite, and the carbon dioxide adsorbent is zeolite.